Evaporative Emissions System: Description and Operation

EVAP System:

Description

The Evaporative Emission (EVAP) control system limits the fuel vapors from escaping into the atmosphere. The EVAP transfers the fuel vapor from the sealed fuel tank to an activated carbon (charcoal) storage device (EVAP canister). The EVAP canister stores the vapors until the engine is able to use the extra fuel vapor.

When the engine is able to use the extra fuel vapor, the intake air flow purges the fuel vapor from the carbon element, and then the normal combustion process consumes the fuel vapor.

The system is required in order to detect the evaporative fuel system leaks as small as 0.040 inch between the fuel filler cap and the purge solenoid. The system can test the evaporative system integrity by applying a vacuum signal (ported or manifold) to the fuel tank in order to create a small vacuum.


Operation

The Control Module then monitors the ability of the system to maintain the vacuum. If the vacuum remains for a specified period of time, then there are no evaporative leaks, and a PASS report is sent to the control module. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot be maintained. Usually a fault can only be detected after a cold start with a trip of sufficient length and driving conditions to run the needed tests. The enhanced evaporative system diagnostic conducts up to e specific sub-tests in order to detect the fault conditions. If the diagnostic fails a sub-test, the Control Module stores a Diagnostic Trouble Code (DTC) in order to indicate the type of fault detected.

The EVAP system uses a switch located in the purge line between the canister and the purge valve in order to detect when the purge is occurring. This switch senses the flow from the engine through the purge valve. When no purge is present, the switch is closed, applying a 12 volt signal to the control module as a NO PURGE signal. When canister purging occurs, the switch opens, turning off the 12 volt signal to the control module.

A scan tool display will indicate that purge is occurring. Clogging of the canister fresh air vent could allow the purge hose between the switch and canister to trap vacuum with the purge valve closed. This would result in a diagnostic indication of a purge valve stuck open or a vacuum switch failure.

Similarly, leaks or blockages in the purge hoses may result in misdiagnosis of the purge valve or vacuum switch. When servicing a purge valve diagnostic trouble code, check the canister fresh air vent, vacuum switch and the integrity of all purge hoses prior to servicing the valve.